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178 Ophthalmic Lenses

Heat toughened Glasses
Heat toughened glass lenses are usually made from spectacle crown glass.
The toughening process begins when the edged lens is placed in a furnace
and heated to 637°C for 50-300 seconds. The time spent in the oven depends
on the weight, size and average thickness of the lens. After heating, the
lens is withdrawn and cooled rapidly, usually by a jet of cold air. The sudden
cooling creates a state of compression at the lens surface and a state of

Fig. 12.3: Toughened lenses are impact resistance lenses. Normal glass lenses
are processed to rearrange their molecular structure to withstand higher impact

Fig. 12.4: Birefringence pattern of heat toughened glasses